pkg_add fails with "fatal error during execution: getcwd"

Hi all,

For the last couple of months, I've seen the following issue on my
-CURRENT laptop:

rho# pkg_add -r libgtkhtml fast-user-switch-applet deskbar-applet
Fetching ftp://ftp.freebsd.org/pub/FreeBSD/ports/i386/packages-8-current/Latest/
libgtkhtml.tbz... Done.
Fetching ftp://ftp.freebsd.org/pub/FreeBSD/ports/i386/packages-8-current/Latest/
fast-user-switch-applet.tbz...pkg_add: fatal error during execution: getcwd
rho#

I've determined that the failing getcwd is the one on line 126 of
src/usr.sbin/pkg_install/lib/pen.c.  I've never seen it fail when trying
to install a single package at a time, only multiple.

Running the pkg_add command again immediately will usually work, or fail
later on a different package.  This started after an upgrade from early
April to late September HEAD, but due to various other changes, I was
unable to revert to the April kernel to test.

I cannot recreate this on any other machine.  My laptop is a Core 2 Duo,
however I can recreate the issue with a non-SMP kernel.  I've forced an
fsck of all filesystems, with no difference.  There's plenty of disk
space left.  I've even deleted all packages and cleared out /usr/local
but still see the problem?

Any suggestions?  Can anyone else recreate this?  As I first saw this
just before RELENG-7 was branched, I'm concerned that it may also affect
that branch.

Gavin